Sounds like they already have a group.
8 is young to start roleplaying. I know people who have done it, but you may need to keep things a little simpler. If they are using adventures/modules I suggest you look at those first. The Core rulebooks themselves are usually pretty tame, but adventures (like books) range in appropriateness and there are some that have adult concepts in them.
If you are looking for a product aimed at a younger audience paizo recently released the Beginner Box
If it is not Pathfinder (or D&D) they are playing please shout out and we can try to tell you what the game is like. "Spirit of the Century" is pretty safe for kids that age. "Werewolf: the Apocalypse" probably isn't.